Multi-gigabit wireless communications at millimeter-wave frequencies are emerging as an important technology for future ubiquitous communications networks. This can be attributed partly to an ever-increasing demand for bandwidth and scarcity of the wireless spectrum, and partly to the decreasing cost of millimeter-wave monolithic integrated circuits (MMIC) which make transmission and receiving devices cheap to produce.
While known fiber optic data transfer devices can provide multi-gigabit per second (Gbps) data rates, infrastructure costs are high and deployment can take years. The wireless alternative is very attractive due to its low cost and rapid deployment. Currently, commercial mm-wave devices are focused on wireless personal area network (WPAN) applications at 60 GHz. These devices are analogue, and typically implement amplitude shift keying (ASK) modulation or binary phase shift keying (BPSK) modulation by controlling the bias voltage of a Gunn-diode oscillator. However, the 60 GHz band is suitable only for short range communications (e.g. indoor) because of higher propagation loss due to oxygen absorption of the propagated radio waves in this band.
Point-to-point wireless links in the at millimeter-wave spectrum currently exist, and use transmitters and receivers implementing modulation techniques such as ASK or BPSK, but have poor characteristic spectral efficiencies of below 1 bit per hertz, and are limited to speeds only up to 1.25 gigabit per second (Gbps).
The recently allocated 71-76 and 81-86 GHz bands in the USA and Europe provide impetus for wireless links with longer range and higher data rates. A practical difficulty is that wireless digital communications devices can not yet achieve such high frequencies. On the other hand, in the analogue domain, real analogue-to-digital (A/D) converters/filters and digital-to-analogue (D/A) converters/filters can not operate at multi-Gbps. While multi-channel approaches may overcome this issue, frequency-domain multiplexing using analogue filters requires frequency guard bands between adjacent radio channels, which is an inefficient use of the available bandwidth. An example is given in Brankovic, et al “High Data Rate Wireless System Solution: 60 GHz/5 GHz Dual Frequency Operation”, 11th IEEE International & Symposium on Personal, Indoor and Mobile Communications, 2000. The Brankovic system uses an OFDM approach with 32 MHz guard bands.
The present invention seeks to overcome or at least reduce one or more of the forgoing problems, or to provide an alternative.